A process for calibrating a gas sensor is known from US 2011/0197649 A1. Provisions are made there for a volume, in which a gas to be measured is circulated through a gas sensor by means of a pump, to be able to be closed by means of a plurality of valves. The closed volume guarantees known conditions as the basis for calculating a concentration.
Processes for calibrating a gas sensor, which are based, in brief, on the release of a known quantity of substance, are described in WO 2007/087403 A2 and U.S. Pat. No. 6,632,674 B1.
In prior-art, so-called diaphragm-controlled gas sensors, the measured signal of the gas sensors depends on the transportation of a substance through a diaphragm of the gas sensor. The diaphragm and its sealing elements separate the sensor system proper and an electrolyte in the interior of the sensor from a respective measured gas and prevent the escape of the electrolyte up to a pressure difference determined by the construction.
Typical calibration processes predefine a certain test gas concentration and check whether the sensor calculates a measured value that is correct in a broad range. A difference between a resulting expected value based on the known test gas concentration and the respective measured value is used here for the correction. The test gas concentration is ideally in a range to be monitored during the operation of the gas sensor or is subject to normative requirements.
A changing pressure acting on the separating diaphragm of the gas sensor leads to a pressure-dependent passage of substance through the diaphragm (increased passage of substance at high pressure; reduced passage of substance at low pressure). Therefore, gas sensors whose principle of function is based on a concentration-dependent passage of substance, therefore show elevated measured values at high pressure. Pressure changes that markedly influence (distort) the measured values of a gas sensor are frequently caused by a pump, which transports the gas to be measured to the gas sensor via a rather long feed line (for example, tube). Such a pump often comprises a rotatingly driven, elastic chamber volume or a piston-and-cylinder unit, both of which operate cyclically and consequently cause cyclical pressure variations.
In addition to the pressure change, which leads to a distorted measured value, a change in the gas concentration is another source of error. The calibration of a gas sensor is adapted to an essentially stagnant medium in an area (gas space) in front of the diaphragm. A so-called depletion layer with a lower concentration of the gas to be measured develops in the immediate vicinity of the diaphragm in a stagnant medium. The molecules to be measured, which pass through the diaphragm due to the concentration gradient, are missing on the opposite side of the diaphragm and are equalized by diffusion from the gas space in a short time. There is a lower concentration directly in front of the diaphragm than in the gas to be measured. If energy is now introduced into the space directly in front of the diaphragm, this concentration stratification is changed. The input of energy results, for example, from a motion of the gas caused by a (varying) volume flow.
The respective prevailing pressure and/or volume flow conditions consequently influence the measurement result. On the one hand, an incorrect measured value is displayed. On the other hand, a change in concentration is displayed, even though the gas concentration has remained the same.